The present invention relates in general to catalytic oxidation of a hydrocarbon feed stock and in particular to preparation of a vanadyl hydrogen phosphate being a useful precursor of vanadium phosphate catalysts for the selective oxidation of hydrocarbons. Vanadium phosphates are important catalysts for the selective oxidation of hydrocarbons in particular in the selective oxidation of butane to maleic anhydride. One of the species showing high catalytic activity is vanadium pyrophosphate ((VO)2P2O7), which is usually formed by a thermal transformation of VOHPO4.xc2xdH2O. Vanadyl phosphate can be prepared using known methods as disclosed in C. J. Hutchings, C. J. Kiely, M. T. Sananes-Schulz, yA. Burrows and J. C. Volta, Catalysis Today 40 (1998) 273. A first method (VPA) comprises of reduction of V2O5 with aqueous HCl followed by conversion with H3PO4 to give VOHPO4.xc2xdH2O. The second method (VPO) involves reduction of V2O5 with isobutanol in the presence of H3PO4. Another process (VPD) is related to a two-step procedure, in which VOPO4.2H2O is prepared from V2O5 and H3PO4 in aqueous medium followed by a reduction of VOPO4.2H2O to VOHPO4.xc2xdH2O in isobutanol. The contemporary art involving aqueous methods involves not only HCl as reducing agent, but e.g. oxalic acid (DE Patent No. 19,645,066 A 1), H3PO3 (DD Patent No. 256, 659 A 1) and hydrazine (JP Patent No. 59,132,938) as well. The general object of this invention is to provide an improved catalyst for the catalytic oxidation of hydrocarbons and a method for the preparation of vanadyl hydrogenphosphate hemihydrate (VOHPO4.xc2xdH2O), vanadyl hydrogenphosphate monohydrate (xcex2-VOHPO4.H2O) and trisvanadyl diphosphate pentahydrate ((VO)3(PO4)2.5H2O). VOHPO4.xc2xdH2O prepared according the present invention is a useful precursor in the preparation of vanadium phosphate catalysts, while xcex2-VOHPO4.H2O and (VO)3(PO4)2.5H2O require conversion to VOHPO4.xc2xdH2O prior to catalysis. The precursors are prepared in formic acid/water mixtures being capable to reduce vanadium (V) to vanadium (IV). Generally, the use of formic acid in the preparation of a vanadium phosphate oxidation catalyst is known in the art as disclosed in e.g. U.S. Pat. No. 4,016,105. Here, the use of formic acid is described in combination with a secondary alcohol. A different system is disclosed in JP Patent No. 53,060,391 describing formic acid water mixtures containing Fe compounds, preferentially in combination with a carrier. Similarly, U.S. Pat. Nos. 4,388,221, 4,481,363, 4,562,269, 4,599,477, 4,639,530 and 4,801,567 describe reduction of V(V) with formic acid water mixtures containing Sn(II) with the aim of preparing supported catalysts for the oxidation of C4 to C10 hydrocarbons to maleic anhydride (U.S. Pat. Nos. 4,388,221, 4,481,363, 4,562,269, 4,639,530, 4,801,567) or supported catalysts for oxidative dehydrogenation of C4 to C8 mono-olefins (U.S. Pat. No. 4,599,477). A third preparation method (U.S. Pat. No. 4,179,404) involves formic acid in a less than stoichiometric amount in order to prevent 100% reduction of V(V) to V(IV). Another methodology is disclosed in EP Patent 71,140. The preparation of the catalyst precursor involves two steps: the synthesis of a vanadium (V) phosphate followed by reduction of V(V) to V(IV) using e.g. formic acid water mixtures affording a mixed vanadium phosphorus oxide. The description of prior art shows that there is an ongoing effort to develop oxidation catalysts by reduction of V(V) to V(IV) with organic and inorganic reducing agents. As a rule, the prior art has avoided the use of formic acid water mixtures, without any co-reducing agent in order to transform V(V) compounds in the presence of phosphorus compounds to crystalline vanadium (IV) phosphate hydrate compounds ((VO)aHb(PO4)c.dH2O; a=b=c=1, d=xc2xd; a=b=c=d=1; a=3, b=0, c=2, d=5). A further object of the invention relates to the application of these vanadium (IV) phosphate hydrate compounds as precursor to the oxidation of butane to maleic anhydride. The precursor is either used as prepared or, if applicable, converted to VOHPO4.xc2xdH2O.
Based on the above observation, this invention is a process for the preparation of a vanadium phosphate catalyst precursor for use in selective oxidation of a hydrocarbon feed stock comprising treatment of a vanadium compound with formic acid, water and a phosphorus compound.